The network for GSM is called PLMN (Public Land Mobile Network). It is connected to the Public Switched Telephone Network (PSTN), but also directly to other mobile networks (UMTS, LTE) and those of other operators.
Communication between the mobile phone and the base station (Network Management System). The TDMA (multiple access time division or Time Division Multiple Access) and FDMA multiple access (frequency division or for FDMA Frequency Division Multiple Access) is used to allow more users to be connected without saturating the network.
Characteristics of the radio interface. GSM uses two frequency bands, one for the uplink (TX) and the other for the downlink (RX) which is part of the control channels, the signal is modulated according to the distance between the GSM antenna.
The 880-915 MHz band is used in Europe for the uplink, while the 925-960 MHz band is used for the downlink.